Research and development of biodegradable or biocompatible synthetic polymers have been progressing in recent years. It is known that biodegradable or biocompatible synthetic polymers greatly vary in characteristics depending on the type of monomer used to constitute the polymer.
For example, polylactide, i.e., polylactic acid, which is a homopolymer of lactide (LA, a cyclic compound formed by dehydration condensation of two molecules of lactic acid) has characteristics including 1) rapid degradation and absorption in the body, 2) low drug permeability, 3) low biodegradability in soil, and 4) hydrophilicity. In contract, poly(ε-caprolactone), which is a homopolymer of ε-caprolactone (CL), a kind of lactone, has characteristics including 1) slow degradation and absorption in the body, 2) high drug permeability, 3) high biodegradability in soil, and 4) hydrophobicity.
There is a known technique of copolymerizing two or more kinds of monomers under various reaction conditions, thereby producing a copolymer having a combined nature of the homopolymers of these monomers, or a copolymer having an intermediate nature of the homopolymers of these monomers.
It has been known that, in the known copolymerization method of lactide and ε-caprolactone, due to the difference in the reaction rate of the respective monomers in the reaction system, polymerization of either lactide or ε-caprolactone proceeds first, and then polymerization of the other proceeds, thereby producing a block copolymer in which the polylactic acid moieties and the poly(ε-caprolactone) moieties are disproportionately present in the copolymer. A recent report disclosed an example of a production method for a lactide-ε-caprolactone copolymer in which the distribution of the monomers is controlled in the copolymer (Patent Document 1). This document uses a predetermined aluminum-salen type complex as a catalyst for the copolymer production.
Further, using a carbon compound of molybdenum as a catalyst in the production of a homopolymer of lactone, such as ε-caprolactone, has also been reported (Patent Document 2).